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DonCesar Hotel, St. Petersburg Beach, Florida
Sunday, Oct. 31st, 1999, 8:30am - 4:30pm
Session Chairs: Stan Heyer - ICC Bob Rosevear - Cigre SC 21
This report summarizes the views of both ICC and CIGRE SC21 regarding the current and future trends in underground ac cable system technology at voltage levels of 60kV and above. Traditional technologies (paper/oil or gas insulation systems) together with more recent technologies (extruded and PPL laminar insulation) are considered as well as future engineering solutions such as superconductivity cable systems and GIL. The report also serves to raise a number of questions and issues in order to encourage contributions and discussion from the participants.
Self contained fluid filled, high pressure oil filled and high pressure gas filled (pipe type) cables continue to be used for some applications including EHV transmission and in extending existing underground cable circuits. Extension of existing cable circuits with the new technologies will require "transition" arrangements.
Question 1: What is the availability of transition joints and what is the service experience of these accessories to date?
PPL insulation is now used extensively at the EHV levels (above 300kV). Extruded insulation is used extensively up to voltage levels of 300kV with several examples also at higher voltage levels. In the future it is anticipated that, with an increasing positive service experience, the trend towards extruded insulation and away from pressurized cable technologies will continue. More extensive use of extruded insulation for ac submarine applications is also expected at voltages up to 150kV. XLPE is the most widely used insulation although Polyethylene and EPR are also still being used up to 230kV and 150kV respectively.
Question 2: Is there any reason to believe that these trends will not continue and what are the experiences? Is XLPE the best solution?
In the ongoing requirement to reduce the cost of underground cable systems without compromising performance, particularly in the voltage range up to 150kV, reduction in insulation thickness, and alternative constructions to extruded metallic sheaths are being commercialized. Trends in accessories include the adoption of premolded and composite joints, which are manufactured in the factory and pre-tested before sending to site. As regards terminations, dry (non-fluid filled) designs are being put into service and alternatives to porcelain insulators are emerging.
Question 3: Are there any important examples of novel cable or accessory designs that have been introduced into service?
Joints remain the perceived risk to performance and reduction in the number of joints within a circuit is viewed as a significant advantage. As a consequence the supply of longer cable lengths will service to reduce this technical risk and potentially reduce overall costs. The logistical problems associated with the manufacture and transportation and installation of longer lengths can be overcome. On line partial discharge testing of accessories can also provide a continuous monitoring facility. Ac commissioning tests also serve to identify potential problems.
Question 4: What experience exists to date in the supply and installation of long lengths and what are the main constraints?
Question 5: What are the test limitations of pd testing of transmission cables and what is the correlation between test data and service performance?
Utilities are striving to achieve better utilization of their asset whilst ensuring that the integrity and long term reliability is not affected. "On line temperature monitoring" can provide a very useful tool.
Question 6: Are "Real Time Ratings" being used and do such facilities offer any other significant benefits?
Some technical issues continue to occupy the attention of the manufacturers which should result in future improvement in performance of the EHV cable systems; ac resistance of large conductors, thermomechanical performance of the cable system; improvement in quality and consistency of quality; simplification of cable (special bonding) systems; reduction in insulation coordination requirements for big cables.
Although still in the development and demonstration stage, High Temperature superconducting (HTS) will find applications for bulk power distribution and transmission and for specific applications. GIL technology is also being considered as a viable alternative.
Question 7: What are these applications and what are the challenges to be addressed before it becomes a commercially viable option?
Question 8: Is GIL a real viable and reliable technology and what are the issues?
Session Chairs: T. Rodenbaugh - ICC Y. Maugain - Cigre SC 21
This report summarizes the views of both ICC and CIGRE SC 21 regarding the laying and installations techniques of underground extruded and SCFF cables at voltage levels of 60 kV and above. The report also serves to raise a number of questions and issues in order to encourage contributions and discussion from the participants.
The cable installation is composed of two main aspects:
· laying techniques which deal with the civil works, · installation techniques which include the pulling and the installation in final position.
Throughout the world, utilities lay more extruded cables than SCFF ones - 93% extruded cables against 46% SCFF. Twelve different existing installation techniques were identified. They are: Trenches, Ducts, Troughs, Tunnels, Micro tunnels, Shafts, Bridges (inside or outside), Mechanical Laying, Horizontal Drilling, Pipe Jacking, Embedding, Use of existing structures. Among them, only three are commonly used (figures coming from Cigre questionnaire sent to utilities). These are : trenches (direct burial), ducts and tunnels.
Question 1 : This shows that the cable industry tend to use only proven techniques. That are the reasons to limit the use of the other techniques : price, safety, threat to less reliability ?
Historically the use of cable installations at extra high voltage has only been considered for special situations where overhead lines are not possible or practical. In recent years, the environmental impact of overhead lines and electromagnetic field issues have become the focus of general public concern and this has resulted in cable systems being considered in areas where previously overhead lines would have automatically been used.
Question 2 : Do you consider that this tendency is only in relation with the cost differential between overhead line and underground cable ? What are the other factors?
The use of existing structures is a promising solution in case of old pipe type cables having to be replaced.
Question 3 : What are the main constraints for laying new XLPE cables in old pipes ? What is your experience?
Question 4: Do you consider this solution as an important trend for the future?
Question 5 : Apart from minimum traffic disturbance during installation, what other aspects are considered in the design of high voltage underground cable systems in public areas ? Is a life cycle assessment covering the installation, maintenance and recycling considered? Will such life cycles assessment be imposed in the future ?
Question 6 : What do you consider as the most efficient way to reduce installation cost : to install cables yourselves to have turnkey contracts, or to separate the cable supply, the civil works, the pulling and the jointing?
Horizontal drilling of PE pipes for major roads, railways and other obstacles crossings, when an open trench is impossible, is adopted by the majority of the utilities.
Question 7 : Are there any technical problem in using this technique?
Emphasis is placed by local authorities upon the reduction of civil works duration in the streets.
Question 8 : What are the solutions you proposed to cope with this problem ?
Question 9 : Do you consider that this will lead to consider new installation techniques more than traditional ones?
According to Cigre questionnaire, utilities throughout the world slightly prefer flexible cable systems to rigid ones for installations in open air (tunnels, ducts).
Question 10 : In your opinion do you consider that this position will change with the installation of more extruded cables in the future ?
Question 11 : When installing cables in ducts, the questions of clearance and side wall pressure are always raised. Do you consider that the problem has a different solution depending on the type of cable laid, SCFF or extruded?
Question 12 : To limit the impact on the environment, it appears that longer cable lengths are required. Lengths up to 1500 m of HV cable are already used. Do you consider that there is a limit?
Short circuits may appear on underground lines coming from internal or external problems. Question 13 : Do you consider the influence of these short circuits towards public (e.g.earth cover over direct buried cables) or equipment (e.g. bridges, tunnels) in the design of the link?
Session Chairs: John Cooper - ICC Willem Boone - Cigre SC 21
This report summarizes the views of both ICC and CIGRE regarding the present situation and in particular future trends in testing techniques of both paper insulated and extruded underground cables at voltage levels of 60kV and above. The report also serves to raise a number of questions and issues to encourage and to structure contributions and discussions from the participants.
· To check the quality of the cable system in order to indicate present or future problems
· To estimate remaining life of the cable system
· To identify liability issues between manufacturer and utility and between utility and customer
· Factory testing (type, routine, sample test)
· Laboratory testing (accelerate aging, development test, prequalification test)
· Field testing (test after installation, test after repair or after circuit modifications, diagnostic test)
The following discussion items and related questions have been prepared. Questions 1 - 4 deal with factory testing and tests after installation (almost always performed on new cable systems), and remaining questions 5 - 10 are related to diagnostic testing. This more novel way of testing is always applied on "old" cable systems and is becoming relevant in relation to predictive maintenance to improve system reliability and to reduce maintenance costs.
Question 1: Will the traditional factory testing after production gradually disappear and be replaced by Quality Assurance Procedures?
Question 2: How necessary will testing become in relation to liability (manufacturer versus utility and utility versus customer)
Question 3: Up to now the test after installation was performed by using DC voltage. Presently the far more effective AC voltage testing is recommended, in particular for extruded cable systems. Will for this type of cable systems, future development go into the direction of non-destructive PDD?
Question 4: Is it meaningful to perform AC-test after installation at AC rated voltage or should transportable equipment be used to perform test at higher voltages and what specific level can be recommended?
Question 5: How important will diagnostic testing become with respect to predictive maintenance and related reliability requirements? How prepared are the utilities already and what are their needs?
Question 6: How sophisticated will diagnostic testing become in the future? Will it be performed on-line, on a basis of monitoring and will it make use of pattern recognition and artificial intelligence (expert system) instead of human intelligence?
Question 7: Many utilities have old impregnated-paper transmission cables that have exceeded their normal life. What diagnostic test methods are available to give a good assessment off loss of life or remaining life and what are the experiences so far?
Question 8: HV cable system field testing appears to mean in practice HV accessory testing, as usually the cable has been tested thoroughly in the factory. What data can be given from practical experience to support this point?
Question 9: The major difference between PD diagnostic testing and other type of diagnostic testing is the ability to locate potential failures. How relevant is this issue if the majority of detected defects is related to accessories, requiring necessarily to locate the potential failure and consequently to use PDD?
Question 10: Commonly a diagnostic test predicts a risk of failure and not remaining life time. How important is this issue for the utility and should more R&D efforts be focused on this point for the coming years to find a solution?
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